For the universal mobile telecommunications system (UMTS) network, the long-term evolution (LTE) has been specified for further enhanced data rates and lower delay (see Non-Patent Literature 1). The LTE-A (also referred to as LTE advanced, LTE Rel. 10, 11, or 12) has been specified for achieving even wider bands and higher speed than those of LTE (also referred to as LTE Rel. 8 or 9), and the succeeding systems (e.g., future radio access (FRA), 5th generation mobile communication system (5G), and LTE Rel. 13) are under study.
LTE Rel. 8 to 12 have been specified assuming exclusive use of frequency bands given to providers (operators) (also referred to as licensed bands). For licensed bands, 800 MHz, 1.7 GHz, 2 GHz or the like is used.
In recent years, the widespread use of smartphones, tablets, or other high-function user terminal (UE) has dramatically increased user traffic. Further additional frequency bands are required to absorb increasing user traffic, although there is a limitation on the number of spectra for licensed bands (licensed spectra).
For this reason, Rel. 13 LTE assumes extending the frequency for LTE systems by using bands of unlicensed spectra (unlicensed bands) which are non-exclusive to licensed bands (see Non-Patent Literature 2). Examples of assumed unlicensed bands include 2.4-GHz and 5-GHz bands in which, for example, Wi-Fi® and Bluetooth® can be used.
In particular, LTE Rel. 13 assumes carrier aggregation (CA) of licensed bands and unlicensed bands. Communication using licensed bands together with unlicensed bands in this manner is referred to as license-assisted access (LAA). It should be noted that future LAA possibly assumes dual connectivity (DC) of licensed bands and unlicensed bands and stand-alone (SA) of unlicensed bands.